A kind of heat processing technique of thinning TiAl alloy cast ingot microstructure
Technical field
The invention belongs to metal material field, particularly a kind of heat processing technique of thinning TiAl alloy cast ingot microstructure.
Background technology
The TiAl alloy is a kind of novel high-temperature structural material, has advantages such as density is low, high-temperature behavior is good, is considered to have aircraft engine of future generation one of structured material of competition potential.Compare with the most sophisticated materials for aeroengines-nickel base superalloy at present, the density of TiAl alloy has only 3.7~3.9g/cm
3, not enough nickel base superalloy density (7.9~9.5g/cm
3) half, this is for the aircraft engine parts, its superiority is self-evident, and, the creeping limit of TiAl alloy is 750 ℃~950 ℃, the anti-oxidant limit is 800 ℃~950 ℃, near 870 ℃~1090 ℃ of 800 ℃~1090 ℃ of the creeping limits of nickel base superalloy and the anti-oxidant limit.Yet its lower temperature-room type plasticity, fracture toughness property and high crack growth rate have seriously hindered it and have entered the process of practicability.Solve an above-mentioned difficult problem, the problem of temperature-room type plasticity difference especially becomes the focus and the key problem of the research and development of TiAl alloy.
At the problem of TiAl alloy temperature-room type plasticity difference, carried out and carried out a large amount of research both at home and abroad.From present present situation, the reason of temperature-room type plasticity difference mainly contains: the order of atomic arrangement and interatomic covalent bonds characteristic in the TiAl alloy; Less and the deformation mechanism complexity of microdeformation mode; Coarse microstructure and interface bond strength are low; Or the like.For this reason, improve its temperature-room type plasticity, become several main research directions at present by alloying, change material preparation and methods such as moulding process, the hot-work of change material and thermal treatment process.Result of study shows that the microstructure of TiAl alloy affects its room-temperature mechanical property significantly, and tiny, uniform microstructure can make alloy in the higher mechanical behavior under high temperature of maintenance, obtains higher room-temperature mechanical property.Further result of study shows, by thermal treatment, or repeatedly forges by high temperature, can change the structural state of TiAl alloy cast ingot effectively, makes its microstructure obtain refinement effectively.The heat treating method of having reported mainly contains cycle heat treatment, two warm processing etc., it mainly acts on is to obtain more even, tiny microstructure, and cost is lower, but shortcoming is to eliminate dentrite and some casting flaw in the microstructure effectively, as loose, cavity etc.And high temperature repeatedly forges and mainly contains isothermal forging, canned forging etc., it mainly acts on is to access more tiny microstructure, and can eliminate dentrite and some casting flaw effectively, but shortcoming is a Deformation structure extremely inhomogeneous (owing to existing the distortion dead band), therefore causes the less stable of performance.Though can improve the homogeneity of tissue by forging deformation repeatedly, its cost is improved, lumber recovery reduces, and, repeatedly forge and cause the spindle cracking easily.When organizing behind the high temperature forging further carried out the homogeneous microstructure processing again, promptly utilize thermal treatment to come homogenizing to forge when organizing again, though the homogeneity of structure of forging deformation is greatly improved, but meanwhile, the small grains that forging deformation forms will be grown up, and counter its temperature-room type plasticity that makes significantly reduces, near the level of directly heat-treating, the advantage of high temperature forging distortion is cut down, therefore, after the forging again heat-treating process be subjected to suspection.
Summary of the invention
The present invention seeks to solve cycle heat treatment or two warm processing and can not eliminate in the microstructure casting flaw problems such as dentrite and loose, cavity effectively, and solve isothermal forging, the Deformation structure that brings inhomogeneous (existing the distortion dead band) is made in the contour warm forging of canned forging, thereby cause the poor stability of performance, spindle cracking problem occurs easily.
A kind of heat processing technique of thinning TiAl alloy cast ingot microstructure, it is characterized in that before high temperature forging, carry out the thermal treatment of one or multi-channel salt bath quenching+timeliness earlier, and then carry out a high temperature forging technology and can realize the refinement and the homogenizing of cast structure.This salt bath quenching+aging technique is the microstructure of thinning TiAl alloy cast ingot effectively not only, simultaneously, also will have influence on follow-up high temperature forging technology, a little crackle of leaving over when making the purpose of follow-up high temperature forging technology change into further elimination dendritic segregation, casting flaw and salt bath quenching etc., rather than for further thinning microstructure, so, can not cause the even and spindle cracking phenomena of the tissue odds that causes greatly because of deflection.By this technology, the original thick cast structure of refinement effectively not only, and the tissue defects of TiAl alloy cast ingot and the defectives such as inhomogeneity of structure behind the high temperature forging are greatly improved, thereby have improved TiAl alloy cast ingot temperature-room type plasticity and stability thereof.Simultaneously, the deflection when this technology has reduced the number of times of high temperature forging distortion and reduced the high temperature forging distortion makes that TiAl alloy cast ingot rimose risk significantly reduces in the cost of high temperature forging distortion and the forging deformation process.Through above-mentioned technology, the mean diameter of its crystal grain is 50 μ m, and corresponding fiducial probability is 90%, and temperature-room type plasticity (unit elongation) can reach and can be stabilized in more than 2%.
At first salt bath quenching+the aging technique that carries out is by original structure being heated to eutectoid temperature T
αMore than the temperature, make it change the α phase fully into, then, by salt bath quenching, change α into unsettled block γ phase (massively transformed gamma) mutually fully, more then, by ageing treatment, make unsettled block γ that branch take place mutually and parse the α phase.Because α has only a kind of (0001) solid matter face mutually, and γ has four kinds of { 111} solid matter faces mutually, so, when the block γ phase time of the mutually fast cold formation of α, be α → γ, α phase crystal grain can only by its unique solid matter face be transformed into a position to γ phase crystal grain, and when unsettled block γ phase timeliness is decomposed, be γ → α, γ phase crystal grain can by its 4 solid matter faces be transformed into 4 positions to α phase crystal grain.Like this, original thick cast structure just can obtain refinement effectively by the transformation mutually of α γ, and refinement original structure can make that the follow-up high temperature forging deformation technique that carries out becomes easily.Salt bath quenching+aging technique also can replace with cycle heat treatment or two warm treatment process, carries out high temperature forging then one time.With respect to cycle heat treatment technology, i.e. oil quenching or shrend by repeatedly realizes the transformation of α phase → block γ phase and the decomposition of block γ phase, the quenching temperature of salt bath quenching is higher, more than TiAl alloy Cold Brittleness Transition Temperature, thereby can avoid quenching crack effectively; Two relatively warm treatment process are promptly at T
αAnd T
ePhase transformation repeatedly mutually comes thinning microstructure with lath γ mutually by α between the temperature, and salt bath quenching+aging technique can reduce heat treatment time effectively; And relatively-high temperature forging process repeatedly, the follow-up high temperature forging among the present invention, not only being out of shape number of times reduces, and deflection also reduces greatly, and the cost of forging deformation and success ratio all can be greatly improved, and demonstrates tangible positively effect.
The present invention at the composition of TiAl alloy, one is high content of niobium TiAl alloy, the content of niobium scope is 6at.%~8at.%, another is the TiAl alloy of common constituent, the aluminium content range is 45at.%~48at.%.When they when being frozen into the room temperature ingot casting by high temperature fluent metal, following phase transformation (seeing accompanying drawing 1) has taken place: during by liquid phase cools, at first form solid phase α, alpha-crystal will be according to qualifications along its C axle growth, thereby form tangible columanar structure.In process of cooling subsequently, alloy enters α+γ two-phase region, will separate out lath-shaped γ phase from the α column crystal.Keep definite orientation relationship between γ phase and the α phase:
(0001)α//{111}γ,<1120>α//[111]γ
When continuing cooling, eutectoid reaction will take place and form α in α mutually
2The full lath tissue of+γ.α
2Ordering transition takes place and forms in α mutually mutually.Because fixed orientation relationship between γ phase and the α phase is so all γ phase laths will present ingot structure and typically organize anisotropy perpendicular to the direction of growth of α phase column crystal.When speed of cooling increases, will separate out the γ phase of Widmannstatten structure (Widmanstatten) or featheriness tissue (feathery) from α mutually, and the γ phase of massive texture (massive).The γ of massive texture is unstable mutually, branch can take place under heating state parse the α phase.
When ingot casting carries out salt bath quenching, be about to ingot casting and be heated to T
αWhen temperature is above, as-cast structure originally, i.e. (α
2+ γ) full lath tissue is converted into single-phase α and organizes.Because the α in the ingot casting of this composition
2Phase content is less, and therefore, the form of α phase will directly be subjected to the influence of γ phase lath.Because this moment, the formation of α phase was a kind of nucleation process again, the new α phase crystal grain that forms is more tiny, and, because the temperature of this moment is higher, and does not have the second phase pinning, so, if soaking time is longer, the α phase crystal grain of new formation is grown up fast.Two temperature heat-treating methods, the solid-state phase changes when utilizing the phase co-conversion between lath γ phase and the α phase are come the refinement ingot structure.But method of the present invention is with newly formation, less α are cooled fast to eutectoid temperature (T mutually
e) below, i.e. γ+below the α two-phase region, make α be transformed into block γ phase mutually, rather than lath-shaped γ phase.The block γ phase constitution that form this moment will be significantly less than lath-shaped γ phase constitution.Lath-shaped γ at room temperature is a kind of stable structure phase mutually, and block γ at room temperature is a kind of unsettled structure phase mutually, at comparatively high temps (T
αAnd T
eBetween the temperature) during timeliness, block γ will decompose mutually, separate out the α phase.Owing to be at T
αSeparate out below the temperature, so the α that separate out this moment is difficult to grow up mutually, and, owing to separate out the α phase time mutually from γ, γ phase crystal grain can by its 4 solid matter faces separate out 4 positions to α phase crystal grain, and do not resemble the block γ phase time of the mutually fast cold formation of α, α phase crystal grain can only by its unique solid matter face be transformed into a position to γ phase crystal grain such, so, the α phase crystal grain that timeliness is separated out will original tiny block γ phase crystal grain be refined as again 4 positions to the zone.If organizing of starting ingot is comparatively thick, can be by twice or the repeatedly in addition refinement of method of salt bath quenching+timeliness.
Through above-mentioned processing, original thick ingot structure will obtain refinement effectively, but the casting flaw of this moment, and because the fast cold defective of bringing into of salt bath quenching will be present in the heat treatment on microstructure, therefore, also need a high temperature forging distortion, further improve and complete tissue.The temperature range of forging deformation is 1230 ℃~1250 ℃, is in γ+α two-phase region, thus this moment do not organize and can obviously grow up.Owing to the refinement of organizing of this moment, therefore, the number of times and the deflection of needed high temperature forging distortion will reduce and reduce.Like this, the homogeneity of fine tissue will can not be subjected to tangible influence, and the high temperature forging distortion of appropriateness also helps the further refinement and the homogenizing of tissue, guarantees the stability of its room-temperature property.
The advantage of salt bath quenching+timeliness, be exactly that it is not as two warm treatment process, by the long time (being generally 18~48 hours), utilize the transformation mutually between lath γ phase and the α phase to come the refinement ingot structure, also not as cycle heat treatment, realize that by oil cooling or water-cooled repeatedly α arrives the transformation of block γ phase and the decomposition of block γ phase mutually, the easy quenching crack that causes like that, but realize that by salt bath quenching α arrives the decomposition of the transformation of block γ phase and block subsequently γ phase mutually.Because the salt temperature scope at 800 ℃~820 ℃, is higher than the Cold Brittleness Transition Temperature (being generally 500 ℃~700 ℃) of TiAl alloy, so the TiAl alloy during salt bath quenching at this moment still is in plastic state, salt bath quenching can not cause quenching crack.And, will be near T
αThe above α of temperature is as cold as T mutually soon
eWhen eutectoid temperature is following, promptly about below 1100 ℃ the time, can realize that fully α arrives the transformation of block γ phase mutually, it is simple to make that the step that realizes this operation becomes, control easily.Aging time after the salt bath quenching was generally 1~2 hour, therefore, its cost was reduced greatly, and economic worth is considerable.
This kind technology not only is suitable for the TiAl alloy cast ingot of common constituent, and (the Al content range is 45at.%~48at.%), and (the Nb content range is 6at.%~8at.%) to be suitable for the TiAl alloy cast ingot of high content of niobium; Not only be suitable for the TiAl alloy cast ingot of common induction melting moulding, and be suitable for the TiAl alloy cast ingot of consumable smelting and ionic fluid melting moulding.
Description of drawings
Fig. 1 is the Ti-Al binary phase diagram
Embodiment
To through cast or scull or lift the TiAl alloy cast ingot of moulding, carry out salt bath quenching+ageing treatment, and the distortion of high temperature forging subsequently, its embodiment is as follows:
(1) the TiAl alloy cast ingot is heated to more than the T α temperature about 10 ℃~15 ℃, the insulation certain hour.The length of this soaking time is relevant with the gauge of ingot casting, determines the time of insulation according to 1 mm/min.For example, 100 millimeters cylinder ingot casting, then soaking time is 50 minutes.Usually, soaking time is to be no more than 60 minutes for good.
(2) above-mentioned ingot casting is quenched fast in the salt bath, ingot casting is transferred to time the salt bath at 5~10 seconds from process furnace, with the requirement greater than 100 ℃/second of the speed of cooling that satisfies salt bath quenching subsequently, makes α can change block γ phase fully into mutually.Ingot casting is incubated 10~20 minutes in salt bath.The working temperature of salt bath is 800 ℃~820 ℃, and the composition proportion of salt bath is (80%~82%) BaCl
2+ (18%~20%) NaCl (wt.%).
(3) according to the size of ingot casting original grain, can be according to above-mentioned technology, circulation is carried out 2~3 times, is converted into block γ tissue fully to guarantee α crystal grain thick in the original structure.
(4) at T
αTemperature is carried out ageing treatment (about 1300 ℃~1320 ℃) for following 20 ℃~40 ℃.Aging time is according to the difference of the composition proportion of virgin alloy and difference was generally 1~2 hour.Stove is as cold as 1250 ℃ more afterwards, is incubated 4 hours, and air cooling obtains tiny nearly complete lamellar structure to room temperature.
(5) in 1230 ℃~1250 ℃ temperature ranges, carry out canned forging or isothermal forging.The forging deformation amount is not more than 50%, and rate of deformation is 1 * 10
-2~1 * 10
-3/ second.Behind the forging deformation, the air furnace stove of putting into 900 ℃ at once is chilled to room temperature, obtains tiny bifurcation tissue.
Nomenclature and chart
(1) nomenclature
At.%---atomic percent
Wt.%---weight percent
Salt bath quenching---workpiece is inserted the fused saline solution fast from high temperature
Oil quenching---workpiece is inserted the quenching oil fast from high temperature
Timeliness---insulation when workpiece is grown under a certain temperature is so that have second to separate out mutually.
α phase---hexagonal structure Ti phase
α
2---orderly body-centred structure Ti
3The Al phase
γ---orderly square structure TiAl phase
T
α---α begins to separate out the temperature (cooling) of γ phase mutually, or the γ phased soln is in the temperature (intensification) of α in mutually.
T
e---the temperature (α → α of eutectoid transformation takes place
2+ γ)
Cold Brittleness Transition Temperature---alloy is changed into the transition temperature of brittle state by plastic state
Canned forging---with stainless steel or superalloy the ingot casting jacket is got up to heat and forged technology.Forging is at room temperature to carry out.
Isothermal forging---workpiece forges under constant temp
Two warm processing---refer in particular at T
αAnd T
eBetween the temperature, come a kind of thermal treatment process of refinement ingot structure by the transformation mutually between γ phase and the α phase.
Cycle heat treatment---refer in particular to by oil cooling or water-cooled repeatedly, utilize α to come a kind of thermal treatment process of refinement ingot structure to the decomposition of the transformation of block γ phase and block γ phase mutually.
Complete lamellar structure---all by α
2The microstructure that+γ synusia group forms
Nearly complete lamellar structure---by α
2+ γ synusia group and a small amount of microstructure that waits axle shape γ crystal grain to form
Bifurcation tissue---the α that waits axle shape γ crystal grain and about 50% by about 50%
2The microstructure that+γ synusia group forms
(2) T of several alloys
αTemperature
The T of several alloys of table 1
αTemperature
Alloy |
?T
αTransition temperature (℃)
|
?Ti—46Al-2Cr-2Nb-1B |
?1320 |
?Ti-48Al-2Cr-2Nb-1B |
?1370 |
?Ti-48Al-2Cr-2Nb |
?1365 |
?Ti-46Al-4Nb-4Hf-0.1Si-1B |
?1360 |